The field of the invention is magnetic resonance imaging and systems. More particularly, the invention relates to methods for simultaneously measuring T*2 and diffusion of a hyperpolarized gas contrast agent.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the nuclei in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) that is in the x-y plane and that is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mxy. A signal is emitted by the excited nuclei or “spins”, after the excitation signal B1 is terminated, and this signal may be received and processed to form an image.
When utilizing these “MR” signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received MR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The measurement cycle used to acquire each MR signal is performed under the direction of a pulse sequence produced by a pulse sequencer. Clinically available MRI systems store a library of such pulse sequences that can be prescribed to meet the needs of many different clinical applications. Research MRI systems include a library of clinically proven pulse sequences and they also enable the development of new pulse sequences.
The MR signals acquired with an MRI system are signal samples of the subject of the examination in Fourier space, or what is often referred to in the art as “k-space”. Each MR measurement cycle, or pulse sequence, typically samples a portion of k-space along a sampling trajectory characteristic of that pulse sequence. Most pulse sequences sample k-space in a raster scan-like pattern sometimes referred to as a “spin-warp”, a “Fourier”, a “rectilinear”, or a “Cartesian” scan. The spin-warp scan technique employs a variable amplitude phase encoding magnetic field gradient pulse prior to the acquisition of MR spin-echo signals to phase encode spatial information in the direction of this gradient. In a two-dimensional implementation (“2DFT”), for example, spatial information is encoded in one direction by applying a phase encoding gradient, Gy, along that direction, and then a spin-echo signal is acquired in the presence of a readout magnetic field gradient, Gx, in a direction orthogonal to the phase encoding direction. The readout gradient present during the spin-echo acquisition encodes spatial information in the orthogonal direction. In a typical 2DFT pulse sequence, the magnitude of the phase encoding gradient pulse, Gy, is incremented, ΔGy, in the sequence of measurement cycles, or “views” that are acquired during the scan to produce a set of k-space MR data from which an entire image can be reconstructed.
There are many other k-space sampling patterns used by MRI systems. These include “radial”, or “projection reconstruction” scans in which k-space is sampled as a set of radial sampling trajectories extending from the center of k-space. The pulse sequences for a radial scan are characterized by the lack of a phase encoding gradient and the presence of a readout gradient that changes direction from one pulse sequence view to the next. There are also many k-space sampling methods that are closely related to the radial scan and that sample along a curved k-space sampling trajectory rather than the straight line radial trajectory.
An image is reconstructed from the acquired k-space data by transforming the k-space data set to an image space data set. There are many different methods for performing this task and the method used is often determined by the technique used to acquire the k-space data. With a Cartesian grid of k-space data that results from a 2D or 3D spin-warp acquisition, for example, the most common reconstruction method used is an inverse Fourier transformation (“2DFT” or “3DFT”) along each of the 2 or 3 axes of the data set. With a radial k-space data set and its variations, the most common reconstruction method includes “regridding” the k-space samples to create a Cartesian grid of k-space samples and then perform a 2DFT or 3DFT on the regridded k-space data set. In the alternative, a set of radial k-space data can also be transformed to Radon space by performing a 1DFT of each radial projection view.
Certain noble gases can be put into a hyperpolarized state and employed as contrast agents in MR imaging applications, yielding substantial SNR increases over traditional proton MR imaging methods. Imaging methods that employ noble gases in the aforementioned manner are disclosed, for example, in U.S. Pat. No. 6,426,058. Of particular interest, is the use of hyperpolarized gas for imaging the air-filled spaces within the lung. In such an imaging study, a hyperpolarized noble gas such as helium (3He) or xenon (129Xe) is inhaled into the lungs prior to the MRI scan. While the spatial resolution attainable in MR images acquired from hyperpolarized gas studies is less than conventional MR imaging techniques, the sensitivity of MRI to the diffusion of hyperpolarized gases within the lung microstructure provides a mechanism for assessing the viability of lung tissue. Using diffusion weighted MRI (“DWI”), the apparent diffusion coefficient (“ADC”) of a hyperpolarized gas, such as helium-3, in the lung can be determined.
In DWI methods, motion sensitizing magnetic field gradients are applied so that the MR images include contrast related to the diffusion of water or other fluid molecules, such as hyperpolarized gas. By applying the diffusion gradients in selected directions during the MRI measurement cycle, diffusion weighted images are acquired from which the ADC is obtained for each voxel location in the reconstructed image. Hyperpolarized gas molecules diffuse less readily when they are restricted by the microstructure of the surrounding tissues. Hence, in diseases such as emphysema, which is characterized by a breakdown in the alveolar walls of the lung, measurements of the ADC of the inhaled hyperpolarized gas can be employed to assess tissue viability. Diffusion weighted MR imaging methods using hyperpolarized gas have been developed; however, the current techniques employ bipolar diffusion sensitizing gradients. Images acquired with and without these bipolar gradients present are used to determine the ADC of the gas in the lung tissues.
The apparent transverse relaxation, or T2*, for a proton species has also found use for assessing tissue viability. Mapping of T*2 for a proton spin species has been demonstrated using multi-echo projection acquisition (“PR”) techniques. Also, a technique using multiple image acquisitions (each acquired in a new breath-hold and at a different echo time) has been used for T*2 mapping in the lungs using PR methods. However, this method requires multiple breath-holds and the T*2 in the lungs has been shown to be highly dependent on lung inflation volume and, therefore, repeatability between breaths.
It would therefore be desirable to provide a method that can simultaneously measure the diffusion and spin relaxation parameters of a hyperpolarized gas contrast agent. More particularly, it would be desirable to provide such a method that is applicable for a single dose of a hyperpolarized gas contrast agent and can be employed within a single breath-hold by the subject.